Tectonic overpressure at shallow depth in the lithosphere: The effects of boundary conditions

In dynamic geological settings, the total pressure (P) comprises dynamic and (litho)static (P_L) components, which are indistinguishable when pressure is estimated thermodynamically. The difference between the two components is called tectonic overpressure (here defined as TOP = P/P_L), the magnitude of which has been debated for decades in geodynamics. In this paper, we report on a parametric study carried out to evaluate the magnitude of TOP in various geodynamic settings under variable boundary conditions. The TOP in the brittle frictional field is limited by the frictional strength and the cohesion of rocks, and by pore fluid pressure; in the viscous field it is dependent on viscosity and flow configuration. We analyse pressure in the viscous field using the Navier-Stokes' equation with gravity, and conclude that there are some configurations and boundary conditions favourable for the development of high TOP (>2) at relatively shallow lithospheric levels, so facilitating the exhumation of high- and ultrahigh-pressure rocks in subduction channels. This is the case for upward tapering subduction channels, with rigid or deformable walls, and no outlet pressure condition at the bottom. Natural subduction zones are characterized by non-steady behaviour over time and space; therefore, if a subduction zone transiently goes through the favourable conditions mentioned above, the TOP can be >2 at shallow lithospheric levels, and high and ultrahigh-pressure rocks can be rapidly exhumed to the surface by return flow close to the hanging wall.